The recent upsurge in the network traffics has made the multilevel phase shift keying (MPSK) system or quadrature amplitude modulation (QAM) system as a promising candidate for an optical modulation system for enabling optical transmissions of 100 Gbit/s or higher, and development and research for such a technique have been intensified.
In order to demodulate signals modulated with such an optical modulation system, an optical receiver including a 90-degree hybrid is required. Here, the 90-degree hybrid exhibits output forms (patterns) having a different branching ratio, depending upon the modulation state of an MPSK signal or a QAM signal, and is the most important component of an optical receiver.
For example, a 90-degree hybrid having an optical waveguide structure which can be monolithically integrated, is constructed from a 4×4 multimode interference (MMI) coupler 100 having four channels on each of the input and output sides, as illustrated in FIG. 13.
It is to be noted that a 90-degree hybrid to demodulate quadrature phase shift keying (QPSK) signals is depicted in FIG. 13. In FIG. 13, the label “QPSK signal” denotes QPSK signal light, whereas the label “LO” denotes local oscillator (LO) light. The labels “I−” and “I+” denote in-phase component (I-component) with phases displaced by 180 degrees from each other, whereas labels “Q−” and “Q+” denote quadrature phase component (Q-component) with phases displaced by 90 degrees with respect to the I-component. It is to be noted that Q− and Q+ have their phases displaced by 180 degrees from each other.
In this 90-degree hybrid, by selecting, as input channels, two channels at positions asymmetrical with each other with respect to the center axis of the four input-side channels of the 4×4 MMI coupler 100, a relationship of phases displaced by 90 degrees from each other is provided by means of the mode interference inside the MMI region.
However, in this 90-degree hybrid, a pair of optical signals (I-component) having an in-phase relationship with each other is outputted from the outer two output channels of the 4×4 MMI coupler 100, and a pair of optical signals (Q-component) having a quadrature phase relationship with each other is outputted from the two inner output channels of the 4×4 MMI coupler 100. In short, a pair of optical signals having an in-phase relationship with each other is outputted from the outer two output channels spatially spaced away from each other. Therefore, where the 90-degree hybrid is coupled to photodiodes to carry out optoelectronic conversion, optical waveguides unavoidably intersect with each other, which give rise to excessive loss.
In order to eliminate intersecting of optical waveguides to reduce the excessive loss, a 90-degree hybrid including a 2×4 MMI coupler 101, a 2×2 optical coupler 102, and a phase shifter 103, as illustrated in FIG. 14, has been proposed.
It is to be noted that a 90-degree hybrid to demodulate a QPSK signal is depicted in FIG. 14. In FIG. 14, the label “QPSK signal” denotes QPSK signal light, whereas the label “LO” denotes LO light. The labels “I−” and “I+” denote I-component with phases displaced by 180 degrees from each other, whereas labels “Q−” and “Q+” denote Q-component with phases displaced by 90 degrees with respect to the I-component. It is to be noted that Q− and Q+ have their phases displaced by 180 degrees from each other.